The present invention relates to compositions useful as materials for the optics of intraocular lenses (IOLs) and to precursors of such compositions. More particularly, the invention relates to polymer mixtures which are derived from the polymerization of a portion of precursor fluids or formulations and are useful in the optics of IOLs, preferably injectable IOLs. Methods of forming IOLs are also disclosed.
Silicone polymers are known materials and many are produced using platinum-containing catalysts, which catalysts remain a part of the final product. For example, certain such silicone polymers are known to be useful in the production of IOLs. Also, curable fluid, for example, liquid, formulations including monomers which can be cured or cross-linked in the presence of platinum-containing catalysts to form silicone polymers have been suggested for use in forming a solid, transparent synthetic lens upon being injected into the lens capsule of an eye from which the natural lens has been removed. See Wright et al U.S. Pat. No. 4,608,050. Lenses which are formed in the lens capsule of the eye are generally referred to herein as injectable IOLs.
Soft, fast curing, low temperature vulcanization (LTV) silicone gels have been developed that target the physical properties of young human lenses, that is the natural lenses present in human eyes. Such soft, fast curing LTV silicone gels provide injectable IOLs that may potentially accommodate under physiological response.
Precursor fluids of such soft, fast curing LTV silicone gels can be injected to fill the natural capsular bag or a synthetic capsular bag and then solidify or cure (polymerize) in situ to form an injectable IOL. Typically, high molecular weight, high viscosity precursor fluids are used so that the material does not readily leak out of the injection sight while in its fluid state. However, with high molecular weight, high viscosity precursor silicone fluids, injection into the eye can be somewhat difficult. Also, a very low level of cross-link density is required with such high viscosity precursor fluids to achieve a similar stiffness (elastic modulus) as a young human natural crystalline lens. Unfortunately, when cross-link density is reduced, the material becomes "gummy" and non-resilient. Thus, such high molecular weight, high viscosity precursor fluids cannot be used for injectable IOLs which match or resemble the properties of a young human lens.
On the other hand, low viscosity, that is a viscosity of less than about 1000 centipoise, low molecular weight precursor fluids result in polymeric materials that are lightly cross-linked and form resilient, responsive gels, that is gels which recover from deformation very quickly. In fact, such gels can have properties quite similar to the properties of a natural crystalline lens. In addition, these low molecular weight precursors, for example, silicones, are more easily purified to achieve medical grade purity than are the higher molecular weight precursors. Also, such low viscosity precursor fluids do not require excessive pressures for injection. Thus, such precursor fluids can be injected through a hand held syringe.
While low viscosity silicone precursor fluids have desirable properties for use in forming injectable IOLs, they do have some important limitations. The most significant limitation is that, because of their fluidic consistency, such low viscosity fluids readily leak out of the desired injection sight. This limitation has been overcome by implementing a "pre-cure" process where the polymerization or cross-linking reaction is initiated prior to injection. In this manner, the fluid material thickens to a controllable viscosity and is then injected. However, there are two fundamental problems with this technique. First, the shear stresses encountered while injecting the "pre-cured" material through a small cannula result in rough areas in the final polymer which are detrimental to the functioning of the resulting lens. Also, in applications where a very fast curing gel is required, the pre-cure cycle limits the work time available for injection.
There is a continuing need for new injectable IOL materials.